WASHINGTON – Republican senators were looking to go skinny Wednesday as they struggled to repeal key parts of former President Barack Obama's health law.Their definition of skinny, however, is evolving and could change many times before they pass a bill — if they pass a bill.One version of the skinny bill would repeal mandates on individuals to buy health insurance and on large businesses to offer health insurance to employees. It would also repeal a tax on medical devices.It would leave the rest of Obamacare intact, falling well short of Republican promises to dismantle the 2010 law."It's just a vehicle to get into conference [with House Republicans] so it may lead to a broader solution," Sen. Lindsey Graham, Republican from South Carolina, said Wednesday, referring to negotiations with the House on a final bill.The House narrowly passed its version of a health bill in May. If the Senate passes a different version, the House could simply pass it and send it to the president. Or House and Senate leaders could form a conference committee to work out the differences.Several Republican senators said this is the path they foresee, which would put off final decisions on what the legislation would eventually look like."Whatever can get 50 votes will pass, then we will go to conference and the real negotiations begin," said Sen. Bob Corker, Republican from Tennessee.A look at what's next:FAST-TRACK TO SOMEWHEREThe legislation is being debated under fast-track budget rules that allow the Senate to pass it on a simple majority instead of having to clear the 60-vote filibuster threshold required of other legislation. Debate, which started Tuesday, is limited to 20 hours. It is expected to end Thursday,Amendments, generally speaking, are unlimited — and can be offered after debate time has expired in a Washington ritual known as "vote-a-rama." That's when amendment after amendment is voted on in what could be an all-night session on Thursday.AMENDMENTS GALOREUnlike other bills, which typically are debated in ways that limit senators' rights to offer changes known as amendments, the current bill is wide open.Democrats can introduce amendments that would be politically difficult for Republicans to oppose and Republicans can do the same.Majority Leader Mitch McConnell, Republican from Kentucky, suffered an initial setback when the Senate voted to block a wide-ranging amendment to replace Obama's statute with a more restrictive substitute. Nine Republicans voted against the bill, including conservatives like Mike Lee of Utah and moderates like Lisa Murkowski of Alaska.The Senate was scheduled on Wednesday to vote on a bill that simply repeals much of Obama's law without replacing it with anything. That vote is expected to fail as well, leaving Republicans grasping for something they can embrace.PARLIAMENTARY PUZZLEThe special fast-track process, called "reconciliation" in Washington-speak, comes with tricky rules. Amendments that are carefully crafted and fit within the rules can pass on a simple majority vote. But many amendments run afoul of the Senate's Byzantine rules, which mean they can require 60 votes and effectively be blocked by Democrats.Among them is the so-called Byrd rule, named after former Sen. Robert Byrd, Democrat from West Virginia. It's complicated, but the Byrd rule disqualifies some of the GOP's ideas, such as a provision in the pending bill aimed at lowering premiums paid by younger, healthier consumers by allowing insurance companies to increase premiums paid by seniors.The Byrd rule generally blocks provisions that don't affect the federal budget — and blocks provisions whose changes to spending or taxes are "merely incidental" to a larger policy purpose. If such provisions are inserted despite the Byrd rule, any individual senators can knock them out with a point of order.